Difference unit

ABSTRACT

A difference unit may include a first sensor to identify a position of a first mark printed on a first web output from a first web press, a second sensor to identify a position of the first mark of the first web, a third sensor to identify a position of a second mark printed on a second web, and a synchronization engine to determine a synchronization difference between the first web and the second web based on the identification of the first mark and second mark as sensed by the second sensor and the third sensor.

BACKGROUND

An inkjet web press is a high-speed, digital; industrial printing devicethat prints on a continuous media web at speeds of hundreds of feet perminute. A roll of media such as a paper on an unwinding device suppliesthe press with a web which is conveyed through the press along a mediapath. Stationary printheads along the media path may eject droplets ofprinting fluid onto the web to form images. The web may be conveyedthrough a drying area and out of the press through rollers to be rewoundon a rewinding device.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate various examples of the principlesdescribed herein and are part of the specification. The illustratedexamples are given merely for illustration, and do not limit the scopeof the claims.

FIG. 1 is a block diagram of a difference unit, according to an exampleof the principles described herein.

FIG. 2 is a block diagram of a system for synchronizing two web presses,according to an example of the principles described herein.

FIG. 3 is a block diagram of a precision repeat device for maintaining aframe-to-frame size consistency along a print media web, according to anexample of the principles described herein.

FIG. 4 is a flowchart depicting a method for synchronizing two webpresses, according to an example of the principles described herein.

FIG. 5 is a flowchart depicting a method for synchronizing two webpresses, according to another example of the principles describedherein.

Throughout the drawings, identical reference numbers designate similar,but not necessarily identical, elements. The figures are not necessarilyto scale, and the size of some parts may be exaggerated to more clearlyillustrate the example shown. Moreover, the drawings provide examplesand/or implementations consistent with the description; however, thedescription is not limited to the examples and/or implementationsprovided in the drawings.

DETAILED DESCRIPTION

Print fluid used in printing may include a significant amount of waterthat can saturate the web. The moisture content of the web and tensionalong the media path within the press, among other factors, may causethe web to expand, lengthening the web. However, when the web is dried,it may shrink back down to a length below its initial state. Therefore,the length of the web coming out of the press may be different than thelength of web being fed into the press. This media distortion maycomplicate post-print finishing operations performed on the printedmaterial by certain finishing devices.

In some examples, two separate presses and associated media webs may beused to form a printed product. A print finishing device may combinemultiple print media webs to produce one finished product. The finishingdevice may fold, cut, or package the multiple print media webs. Themethods used to combine these multiple print media webs may use verytight control of the length of each print media web fed into thefinishing device. Some print media web presses may not be sufficientlyconsistent to permit two independent presses printing different contenton two separate print media webs to feed the same piece of finishingequipment and maintain synchronization between the two print media websacross tens of thousands of pages or printed frames.

A printing process in a web press may cause distortions in the length ofthe print media web that complicate post-finishing operations in certainfinishing devices, More specifically, the significant application ofmoisture to the media web during printing, followed by the removal ofthat moisture through a drying process results in a variability in printframe length and an overall reduction in the length of the web. Forexample, the media web may shrink at a rate of approximately 0.2%, whichis about 1 foot for every 500 feet of media web fed into a press.

Finishing devices that initiate finishing operations on a fixed indexbasis for each print frame printed on the media web, or, multi-webfinishing devices that combine rolls from different sources, do nottolerate such media distortions effectively. This is because thedistorted media web eventually causes print frames to drift out of thefinishing device's tolerance band, and the finishing operationsincluding folding, combining, and cutting begin to occur within adjacentprint frames rather than between print frames as intended. In order toaccommodate a higher rate of media distortion associated with a digitalinkjet web press, a finishing device initiates finishing operationsbased on triggers from the media or the press. Advanced digitalfinishing devices are available that provide such triggering mechanismsbased on control systems that compensate for the cumulative error in weblength. However, many commercial (and other) print customers who operatedigital inkjet web presses prefer the lower costs and higherproductivity of fixed index finishing equipment. Moreover, many printcustomers who already own such legacy finishing equipment want toleverage it forward rather than incur the significant costs associatedwith acquiring more advanced digital finishing devices.

Some print press systems use methods of dealing with media distortionsthat are based on dynamically measuring the length of the produced pagesand then trying to adjust the frame length to make and keep the lengthclose to its nominal value. However, the mechanisms used to find thelength of the page are based on measuring the speed of the media web ata point that is close to the end of the media path of the media web, andmeasuring the time a page takes to pass through this point. The speed ofthe media web is not constant, and varies during the time a page takesto pass through the point. There is not a definite speed available toconvert time into page length. Determining the precise speed of themedia web may be difficult. The speed of the media web may be derivedfrom a number of marks laid on the media web and read by at least onesensor. However, due to considerations such as the real estateconstraints of the printed page layout, it is not always possible tohave a high enough number of marks on the page to provide an accurateaverage. The speed can also be measured indirectly, for example, bycounting the revolutions of a roll of a known diameter. However, theaccuracy of this measurement may suffer from errors due to paperslippage on the roll, or thermally-induced variations of the diameter ofthe roll. The lack of accuracy in measuring the speed of the media webas it moves through a printing system translates into a lack of accuracyin the measured frame length, which is often outside of acceptableranges for some printing applications. For example, in packaging andother applications where the frames tend to be long, the errorsexperienced may not be acceptable.

This issue is compounded when attempting to also synchronize a pluralityof media webs printed on two separate web presses in preparation forfinishing within the finishing device. A precision repeat (PR) devicemay be used to correct media distortions within a single printing press,but a PR device is not well calibrated from press-to-press. The resultis that a print stream from any one press may be consistent fromframe-to-frame, but print streams are not consistent enough frompress-to-press to permit use of multi-web finishing systems that arecapable of performing finishing operations including folding, combining,and cutting for multiple media webs.

Examples described herein provide a difference unit for synchronizingbetween two web presses. The difference unit may include a first sensorto identify a position of a first mark printed on a first web outputfrom a first web press, a second sensor to identify a position of thefirst mark of the first web, a third sensor to identify a position of asecond mark printed on a second web, and a synchronization engine todetermine a synchronization difference between the first web and thesecond web based on the identification of the first mark and second markas sensed by the second sensor and the third sensor.

The difference unit may include a communication engine to send thesynchronization difference to the first web press and the second webpress. The first and third sensors are located at the same positionalong a web movement direction.

Examples described herein also provide a system for synchronizing twoweb presses. The system may include a first web press to output a firstweb. The first web press includes a first precision repeat device. Thesystem may also include a second web press to output a second web. Thesecond web press may include a second precision repeat device.

The system may also include a difference unit. The difference unit mayinclude a first sensor to identify a position of a first mark printed onthe first web, a second sensor to identify position of the first mark ofthe first web, a third sensor to identify a position of a second markprinted on the second web, and a synchronization engine to determine asynchronization difference between the first web and the second webbased on the identification of the first mark and second mark as sensedby the second sensor and the third sensor.

The first precision repeat device of the system may include a firsttimer to measure a time T1 from a fourth sensor sensing the first markand a fifth sensor sensing a third mark printed on the first web, and asecond timer to measure a time T2 from the fifth sensor sensing thethird mark and the fourth sensor sensing a next first mark. The firstprecision repeat device may also include a first controller to control agap between the frames by regulating when frames are printed on the webbased on T1 and T2.

The second precision repeat device may include a third timer to measurea time T3 from a sixth sensor sensing the second mark and a seventhsensor sensing a fourth mark printed on the second web, and a fourthtimer to measure a time T4 from the seventh sensor sensing the fourthmark and the sixth sensor sensing a next second mark. The secondprecision repeat device may also include a second controller to controla gap between the frames by regulating when the frames are printed onthe web based on T3 and T4.

The synchronization engine communicates the synchronization differenceto the first controller and the second controller. The first controllerand the second controller control their respective gaps based at leastpartially on the synchronization difference. The system may also includea finishing device downstream from the difference unit to perform atleast one post-synchronization operation on the first and second webs.

Examples described herein also provide a method for synchronizing twoweb presses. The method may include, with a first precision repeatdevice of a first web press, dynamically adjusting a size of a gapbetween frames on a first media web, and, with a second precision repeatdevice of a second web press, dynamically adjusting a size of a gapbetween frames on a second media web. The method may also include, witha difference engine, identifying, with a first sensor, a position of afirst mark printed on the first media web, identifying, with a secondsensor, the position of the first mark, and identifying, with a thirdsensor, the position of a second mark printed on the second media web.The method may further include determining a synchronization differencebetween the first media web and the second media web based on theidentification of the first mark and the second mark as sensed by thesecond sensor and the third sensor, and sending a synchronizationdifference to the first web press and the second web press.

In one example, the method may include adjusting a gap between the firstweb and the second web based on the synchronization difference. Further,the method may include, with a first controller of the first web pressand a second controller of the second web press, controlling respectivegaps between frames of the first web and the second web, respectively,based at least partially on the synchronization difference.

Determining the synchronization difference between the first web and thesecond web based on the identification of the first mark and third markas sensed by the second sensor and the third sensor may includecomparing a difference between a time T1 between the first sensoridentifying the position of the first mark and the second sensoridentifying the position of the first mark, and a time T2 between thesecond sensor identifying the position of the first mark and the thirdsensor identifying the position of the third mark. Determining thesynchronization difference between the first web and the second webbased on the identification of the first mark and third mark as sensedby the second sensor and the third sensor may also include determiningif T1 does not equal T2. When T1 is greater than T2, decreasing the gapbetween printed frames of the first web and printed frames of the secondweb. When T2 is greater than T1, increasing the gap between the printedframes of the first web and the printed frames of the second web.

Dynamically adjusting the size of the gap between frames on the firstmedia web with the first precision repeat device of the first web pressmay include measuring a third time T3 between a fourth sensor sensingthe first mark printed on the first web and a fifth sensor sensing thesecond mark printed on the first web, measuring a fourth time T4 betweenthe fifth sensor sensing the second mark and the fourth sensor sensing anext first mark, and adjusting a gap between printed frames when T3 doesnot equal T4. Dynamically adjusting the size of the gap between frameson the second media web, with the second precision repeat device of thesecond web press, may include measuring a fifth time T5 between a sixthsensor sensing the third mark printed on the second web and a seventhsensor sensing a fourth mark printed on the second web, measuring asixth time T6 between the sixth sensor sensing the third mark and theseventh sensor sensing a next third mark, and adjusting a gap betweenprinted frames when T5 does not equal T6. The method may also includedecreasing the gap between printed frames by reducing an amount of timebetween printing sequential frames on a media web, and increasing thegap between printed frames by increasing the amount of time betweenprinting sequential frames on a media web. Adjusting a gap betweenprinted frames comprises determining an error in timing between sensingthe first and second marks, the error according to the followingequation:

error=sign(T1−T2)*min(T1,T2)  Eq. 1

where: sign(x) is 1 if x>0, −1 if x<0, and zero if x=0, and minx, y) isthe minimum of x and y.

Turning now to the figures, FIG. 1 is a block diagram of a differenceunit (100), according to an example of the principles described herein.The difference unit (100) synchronizes two web presses and theirrespective print media webs, and may include a first sensor (101-1) toidentify a position of a first mark (103-1) printed on a first web(150-1) output from a first web press (104-1). A second sensor (101-2)may be included in the difference unit (100) to identify a position ofthe first mark (103-1) of the first web (150-1). The second sensor(101-2) may be located further downstream relative to a direction oftravel of the first web (150-1) as indicated by arrow (180). the sensors(101) described throughout this disclosure may include a scanner, acamera, or other imager, implementing various image sensors such as, forexample, charge coupled devices (CODs) or complementarymetal-oxide-semiconductor (CMOS) devices.

The difference unit (100) may also include a third sensor (101-3) toidentify a position of a second mark (103-2) printed on a second web(150-1). A synchronization engine (102) may be included in thedifference unit (100) to determine a synchronization difference betweenthe first web (150-1) and the second web (150-2) based on theidentification of the first mark (103-1) and second mark (103-2) assensed by the second sensor (101-2) and the third sensor (101-3).

The marks (103-1, 103-2, collectively referred to herein as 103) may be,for example top-of-form (TOF) marks that are printed by the first webpress (104-1) and the second web press (104-2) onto the first web(150-1) and second web (150-2), respectively. The marks (103) may takeany form that the sensors (101-1, 101-2, 101-3, collectively referred toherein as 101) can detect.

The sensors (101) may be any optical sensing device that can detect themarks (103) printed on the first web (150-1) and second web (150-2). Thethree sensors (101) are mounted rigidly within the difference unit (100)in a right triangle with each of the sensors (101) being located at oneof the three vertices of the right triangle. The first sensor (101-1)and the second sensor (101-2) image the first web (150-1), and the thirdsensor (101-3) images the second web (150-2), Each sensor (101) includesa detection path (105-1, 105-2, 105-3, collectively referred to hereinas 105) which is a point at which the sensors (101) detect the surfaceof the media webs (150).

The synchronization engine (102) may receive as input data from each ofthe sensors (101), and acts as a timing circuit that detects the timingbetween the first web (150-1) and the second web (150-2) based on thesensors (101) detecting the marks (103) on the first web (150-1) and thesecond web (150-2). Specifically, the first sensor (101-1) may sense thefirst mark (103-1) as it passes the first sensor (101-1), and the secondsensor (101-2) may sense the first mark (103-1) as it passes the secondsensor (101-2). A time between the first sensor (101-1) sensing thefirst mark (103-1) and the second sensor (101-2) sensing the first mark(103-1) may be determined and indicates the speed at which the first web(150-1) is moving and a frame-to-frame variation along the first web(150-1).

The third sensor (101-3) detects the second mark (103-2) printed on thesecond web (150-2). The objective is to ensure that the first mark(103-1) crosses the detection path (105-2) of the second sensor (101-2)at the same time the second mark (103-2) crosses the detection path(105-3) of the third sensor (101-3). The layout of the sensors (101) ina right triangle assures that the time between the second mark (103-2)crossing the detection path (105-2) of the second sensor (101-2) and thetime the second mark (103-2) crosses the detection path (105-3) of thethird sensor (101-3) is positive and non-zero. There is no ability tomeasure negative time, and like all other real devices, thesynchronization engine (102) of the difference unit (100) measurespositive time. The fixed distance between the second sensor (101-2) andthe first sensor (101-1) may be subtracted from the reading of the timebetween the second sensor (101-2) and the third sensor (101-3) toprovide a synchronization difference. The synchronization difference isto be centered around a constant such as, for example, zero. Thesynchronization difference may then be used to correct the differencebetween the alignment of the first mark (103-1) and the second mark(103-2) so that the first web (150-1) is synchronized with the secondweb (150-1) and their respective printed frames are aligned with oneanother.

In one example, the variance from the constant (i.e., thesynchronization difference) may be divided in half with each halfserving as a correction that can be supplied to the two web presses(104-1, 104-2, collectively referred to herein as 104) so that both webpresses (104) are adjusted to bring the synchronization difference backto the constant and maintain the synchronization difference at theconstant or at least within a threshold. Thus, where the differencevalue is positive, the first web (105-1) leads the second web (150-2),and, thus, the first web (150-1) may slow down and the second web(150-2) may speed up. In other words, frames from the first web (105-1)happen sooner in the print stream, so the frames will be shorter. Wherethe difference is negative the inverse occurs. When there is nodifference, no correction is applied. Likewise, the frames on the secondweb (150-2) will be longer. Shortening and lengthening the frames may beperformed by the web presses (104) during printing, and any changes madeby the web presses (104) are detected at the difference engine (100). Inthis manner, the web presses (104) and the difference engine (100) forma feedback loop. The feedback from the difference unit (100) may beprovided to the web presses (104) often to ensure that thesynchronization difference is continually corrected.

FIG. 2 is a block diagram of a system (200) for synchronizing two webpresses (104), according to an example of the principles describedherein. The system includes the first web press (104-1) to output afirst web (150-1). The first web press (104-1) includes a firstprecision repeat device (203-1). A second web press (104-2) is alsoincluded in the system (200) to output a second web (150-2). The secondweb press (104-2) includes a second precision repeat device (203-2).

Further, located downstream, the difference unit (100) of FIG. 1 isincluded. The difference unit (100) includes the first sensor (101-1),the second sensor (101-2) the third sensor (101-3), and thesynchronization engine (102) as described herein in connection with FIG.1.

Each of the web presses (104) may include an unwinding device (201-1,201-2, collectively referred to herein as 201). The unwinding deviceincludes a spool of print media web that serves as the first web (150-1)and the second web (150-2). Further, each web press (104) may eachinclude a number of printing devices (202-1, 202-2, collectivelyreferred to herein as 202) that print images onto the first web (150-1)and the second web (150-2), respectively.

Each web press (104) may also include a rewinding device (204-1, 204-2,collectively referred to herein as 204). The rewinding device (204) isused to collect the printed-on web (150) onto a spool much like the web(150) existed as it sat on the unwinding device (201) before the web(150) was unspooled from the unwinding device (201). However, becausethe first web (150-1) and the second web (150-2) are fed into thedifference unit as depicted in FIGS. 1 and 2, the rewinding devices(204) may be bypassed.

The precision repeat devices (203) are, like the difference unit (100),feedback devices that provide feedback to the printing devices (202),but within their respective web presses (104). FIG. 3 is a block diagramof a precision repeat device (203) for maintaining a frame-to-frame sizeconsistency along a print media web, according to another example of theprinciples described herein.

The first precision repeat device (203-1) may include a first timer(301-1) to measure a time T1 from a fourth sensor (101-4) sensing thefirst mark (103-1) and a fifth sensor (101-5) sensing a third mark(103-3) printed on the first web (150-1). A second timer (301-2) may beincluded to measure a time T2 from the fifth sensor (101-5) sensing thethird mark (103-3) and the fourth sensor (101-4) sensing a next firstmark (103-1) located on a next printed frame. A first controller (302-1)may be included in the first precision repeat device (203-1) to controla gap between the frames by regulating when frames are printed on thefirst web (150-1) based on T1 and T2. The second precision repeat device(203-2) includes identical elements as described in connection with thefirst precision repeat device (203-1) and are described here withoutdepiction in the figures to avoid repetition. The second precisionrepeat device (203-2) includes a third timer (301-3) to measure a timeT3 from a sixth sensor (101-6) sensing the second mark (103-2) and aseventh sensor (101-7) sensing a fourth mark (103-4) printed on thesecond web (150-2). The second precision repeat device (203-2) alsoincludes a fourth timer (301-4) to measure a time T4 from the seventhsensor (101-7) sensing the fourth mark (103-4) and the sixth sensor(101-6) sensing a next second mark (101-2) printed on the second web(150-2). A second controller (302-2) is also included in the secondprecision repeat device (203-2) to control a gap between the frames byregulating when the frames are printed on the second web (150-2) basedon T3 and T4.

With description now from FIG. 3, the first timer (301-1) measures thetime between these sensing events as time T1. That is, the first timer(301-1) starts counting when the fourth sensor (101-4) senses the firstmark (103-1) in a subsequent frame (i.e., frame n+1), and stops countingwhen the fifth sensor (101-5) senses the third mark (103-3) in the firstframe (i.e., frame n). Likewise, the second timer (301-2) measures thetime between the fifth sensor (101-5) sensing the third mark (103-3) inframe n, and the fourth sensor (101-4) sensing a next first mark (103-1)in the subsequent frame n+1. The second timer (301-2) measures the timebetween these sensing events as time T2.

The first controller (302-1), executing a frame first gap adjustmentmodule (303-1), receives and analyzes times T1 and T2 to determine ifthere is a difference between times T1 and T2. A difference betweentimes T1 and T2 indicates that the distance between the first mark(103-1) and the third mark (103-3) is not the same as the fixed distancebetween fourth sensor (101-4) and the fifth sensor (101-5), which inturn indicates that there is some error, or distortion, in the length ofthe frames. More specifically, when T1 is less than T2, the firstcontroller (302-1) determines that the frame length has undergoneshrinkage, and that the gap should be therefore be increased in size tocompensate for the shrinkage. The error, or amount of time by which thegap is adjusted is the lesser of the two times T1 and T2. The analysisperformed by execution of the first gap adjustment module (303-1) todetermine the correction error is demonstrated by the followingequation:

error=sign(T1−T2)*min(T1,T2)  Eq. 1

where: sign(x) is 1 if x>0, −1 if x<0, and zero if x=0, and min(x, y) isthe minimum of x and y.

In a second scenario where the first web (150-1) has undergoneexpansion, the fifth sensor (101-5) senses the third mark (103-3) inframe n as the first web (150-1) travels along the print path in thedirection indication by arrow (180). Shortly thereafter, the fourthsensor (101-4) sees the first mark (103-1) in frame n+1, The secondtimer (301-2) measures the time between these sensing events as time T2.That is, the second timer (301-2) starts counting when fifth sensor(101-5) senses the third mark (103-3) in frame n, and it stops countingwhen the fourth sensor (101-4) senses the first mark (103-1) in framen+1. Likewise, the first timer (301-1) measures the time between sensorthe fourth sensor (150-1) sensing the first mark (103-1) in frame n+1,and the fifth sensor (101-5) sensing the third mark (103-3) in framen+1. The first timer (301-1) measures the time between these sensingevents as time T1.

The first controller (302-1) receives and analyzes times T1 and T2 for adifference. Again, a difference between times T1 and T2 indicates thatthe distance between the first mark (103-1) and the third mark (103-3)is not the same as the fixed distance between the fourth sensor (101-4)and the fifth sensor (101-5), which in turn indicates that there is someerror, or distortion, in the length of the frames. More specifically,when T1 is greater than T2, the first controller (302-1) determines thatthe frame length has undergone expansion, and that the gap shouldtherefore be decreased in size to compensate for the expansion. Theerror, or amount of time by which the gap is adjusted is the lesser ofthe two times T1 and T2, As in the above example, the analysis performedby execution of the first gap adjustment module (303-1) to determine thecorrection error is demonstrated by Eq. 1 above.

Dynamically adjusting the size of the gap between frames on the firstmedia web (150-1) with the first precision repeat device (203-1) of thefirst web press (104-1) may include measuring a third time T3 between afourth sensor (101-4) sensing the first mark (103-1) printed on thefirst web (150-1) and a fifth sensor (101-5) sensing the second mark(103-2) printed on the first web (150-1), measuring a fourth time T4between the fifth sensor (101-5) sensing the second mark (103-2) and thefourth sensor (101-4) sensing a next first mark (103-1), and adjusting agap between printed frames when T3 does not equal T4. Dynamicallyadjusting the size of the gap between frames on the second media web(150-2), with the second precision repeat device (203-1) of the secondweb press (104-2), may include measuring a fifth time T5 between a sixthsensor (101-6) sensing the third mark (103-3) printed on the second web(150-2) and a seventh sensor (101-7) sensing a fourth mark (103-4)printed on the second web (150-2), measuring a sixth time T6 between thesixth sensor (101-6) sensing the third mark (103-3) and the seventhsensor (101-7) sensing a next third mark (103-3), and adjusting a gapbetween printed frames when T5 does not equal T6. The method may alsoinclude decreasing the gap between printed frames by reducing an amountof time between printing sequential frames on a media web (150), andincreasing the gap between printed frames by increasing the amount oftime between printing sequential frames on a media web (150).

With reference to FIG. 3, FIG. 2 depicts the manner in which the datacreated by the first precision repeat device (203-1) and the secondprecision repeat device (203-2) are used to alter the manner in whichframes are printed on the first web (150-1) and the second web (150-2).The first controller (302-1) of the first precision repeat device(203-1) and the second controller (302-2) of the second precision repeatdevice (203-2) send instructions to the first printing device (202-1)and second printing device (202-2), respectively to adjust gaps betweenprinted frames of print media webs (150) as indicated by arrows (220-1,220-2). In this manner, the first precision repeat device (203-1) andthe second precision repeat device (203-2) serve to reduce or eliminateframe-to-frame variance.

In addition to the feedback provided by the first precision repeatdevice (203-1) and the second precision repeat device (203-2), thedifference unit (100) also provides feedback. As described herein, thesynchronization difference determined by the difference unit (100) isprovided to the first printing device (202-1) and second printing device(202-2), respectively, via communication lines (221-1, 221-2). In oneexample, the difference unit (100) divides the variance into twoseparate halves such that a signal going to the first printing device(202-1) may include correction information instructing the firstprinting device (202-1) to correct its speed in a first direction(faster or slower), and the signal going to the second printing device(202-2) may include correction information instructing the secondprinting device (202-2) to correct its speed in a second direction(slower or faster) opposite to the direction of correction by the firstprinting device (202-1). In this manner, rather than a single one of thefirst printing device (202-1) and second printing device (202-2) makingthe correction, both printing devices (202-1, 202-2) participate incorrecting the variance. In this manner, the difference unit (100)serves to reduce or eliminate stream-to-stream variance between two webs(150).

In one example, the synchronization difference may be communicated bythe synchronization engine (102) to the first controller (302-1) of thefirst precision repeat device (203-1) and the second controller (302-2)of the second precision repeat device (203-2). In this example, thefirst controller (302-1) and the second controller (302-2) may act aspassive intermediary devices that forwards the data representing thesynchronization difference onto the first printing device (202-1) andthe second printing device (202-2), or the first controller (302-1) andthe second controller (302-2) may further process the data representingthe synchronization difference. For example, the first controller(302-1) and the second controller (302-2) control their respective gapsbased at least partially on the synchronization difference. In otherwords, the variance in gaps between printed frames controlled by theprecision repeat devices (203-1, 203-2) may also include instructions tosynchronize the two print media webs (150) using the data representingthe synchronization difference. In this example, the combination of datadefining corrections to the variance in the gaps and the datarepresenting the synchronization difference may be sent to the firstprinting device (202-1) and the second printing device (202-2) for usein correcting these issues. Thus, in some examples, stream-to-streamvariation correction may be added on top of existing precision repeatdevice (203) frame-to-frame variance so that that composite producesboth consistent frame sizes and consistent, synchronized streams.

In another example, the data representing the synchronization differencemay be sent directly to the first printing device (202-1) and the secondprinting device (202-2) separate from the data defining corrections tothe variance in the gaps produced by the precision repeat devices(203-1, 203-2). Further, the small size of the signals and the distancebetween the precision repeat devices (203-1, 203-1) and printing devices(202-1, 202-2) of each web press (104-1, 104-2), and the differenceengine (100) which is mounted relatively closer to the finishing device(205) may affect the transmission of the data representing thesynchronization difference. Thus, the data representing thesynchronization difference communicated to each press via communicationlines (221-1, 221-2) may not be applied all at once. Instead, in oneexample, the data representing the synchronization difference may be fedinto the web presses (104) of the system (200) through a filter. In thisexample, the data representing the synchronization difference may besent via a TOF windowing signal.

In one example, the time-of-flight transit from the first sensor (101-1)to the second sensor (101-2) as depicted in FIG. 1 may be used tocalibrate the distance between the first sensor (101-1) and the secondsensor (101-2). Such a calibration may occur once in the lifetime of thedifference unit (101) or may be perform any number of times before,during, and after operation of the difference unit (100). Further, inone example, signals generated by the first sensor (101-1) and thesecond sensor (101-2) may be used as a heartbeat signal; a periodicsignal generated by the first sensor (101-1) and the second sensor(101-2) and associated synchronization engine (104-2) to indicateoperation or to synchronize other parts of the difference unit (101).

Returning again to FIG. 2, the we press synchronization system (200) mayinclude a finishing device (205) downstream from the difference unit(100) to perform at least one post-synchronization operation on thefirst web (150-1) and the second web (150-2). In one example, thefinishing device (205) may combine the two streams of webs (150) into acommon stack, and perform a number of finishing processes includingcutting, folding, collating, packaging, other finishing processes, orcombinations thereof.

Thus, the system (200) includes two web presses (104-1, 104-2) that runtheir outputs together. Each web press (104) includes a precision repeatdevice (203-1, 203-2) that provides feedback to cause the web presses(104) to print; on average, pages of a fixed size regardless of changesin the size of the webs (150-1, 150-2) due to moisture content, tensionalong the media path of the webs, etc., as printing is performed. Thedifference unit (100) detects differences in frame size between the webs(150), and acts to correct the size differences by operating theprecision repeat devices (203) on each web press (104-1, 104-2). Eachweb press (104) is fed back half of the overall error from thedifference unit (100), with an inverted sign. The media web (150) whichis too short is directed to become longer and the media web (150) whichis too long is directed to become shorter.

FIG. 4 is a flowchart (400) depicting a method for synchronizing two webpresses (104-1, 104-2), according to an example of the principlesdescribed herein. The method (400) may include, with a first precisionrepeat device (203-1) of a first web press (104-1), dynamicallyadjusting (block 401) a size of a gap between frames on a first mediaweb (150-1). The method (400) may further include, with a secondprecision repeat device (203-2) of a second web press (104-2),dynamically adjusting (block 402) a size of a gap between frames on asecond media web (150-2).

A difference engine (100) may be used to identify (block 403), with afirst sensor (101-1), a position of a first mark (103-1) printed on thefirst media web (150-1). The method (400) may further include, with thedifference engine (100), identifying (block 404), with a second sensor(101-2), the position of the first mark (103-1) and identifying (block405), with a third sensor (101-3), the position of a second mark (103-2)printed on the second media web (150-2).

The synchronization engine (102) of the difference engine (101) may beused to determine (block 406) a synchronization difference between thefirst media web (150-1) and the second media web (150-2) based on theidentification of the first mark (103-1) and the second mark (103-2) assensed by the second sensor (101-2) and the third sensor (101-3). Asynchronization difference may be sent (block 407) to the first webpress (104-1) and the second web press (104-2) using the synchronizationengine (102) of the difference engine (101).

FIG. 5 is a flowchart depicting a method (500) for synchronizing two webpresses (104-1, 104-2), according to another example of the principlesdescribed herein. The method (500) may include, with a difference engine(100), identifying (block 501), with a first sensor (101-1), a positionof a first mark (103-1) printed on the first media web (150-1). Themethod (500) may further include, with the difference engine (100),identifying (block 502), with a second sensor (101-2), the position ofthe first mark (103-1) and identifying (block 405), with a third sensor(101-3), the position of a second mark (103-2) printed on the secondmedia web (150-2). The method (500) may further include, with thedifference engine (100), identifying (block 503), with a third sensor(101-3), the position of a second mark (103-2) printed on the secondmedia web (150-2).

The difference between a time T1 between the first sensor (101-1)identifying the position of the first mark (103-1) and the second sensor(101-2) identifying the position of the first mark (101-1), and a timeT2 between the second sensor (101-2) identifying the position of thefirst mark (103-1) and the third sensor (101-3) identifying the positionof the third mark (103-3) may be compared (block 504), and adetermination (Block 505) may be made as to whether T1 is greater thanT2. If T1 is greater than T2 (Block 505, determination YES), then thefirst precision repeat device (203-1) and the second precision repeatdevice (203-2) may decrease (block 506) the gap between printed framesof the first web (150-1) and the second web (150-2), respectively. If,however, T1 is not greater than T2 (i.e., T1 is less than T2) (Block505, determination NO), then the first precision repeat device (203-1)and the second precision repeat device (203-2) may increase (block 507)the gap between printed frames of the first web (150-1) and the secondweb (150-2), respectively.

Blocks 506 and 507 move to block 508, where, a synchronizationdifference between the first media web (150-1) and the second media web(150-2) may be determined (block 508) using the difference engine (100).The method (500) may also include, with the difference engine (100),sending (block 509) a synchronization difference to the first web press(104-1) and the second web press (104-2). The method (500) may include,with a first precision repeat device (203-1) of a first web press(104-1), dynamically adjusting (lock 510) a size of a gap between frameson a first media web (150-1). Further, the method (500) may include,with a second precision repeat device (203-2) of a second web press(104-2), dynamically adjusting (block 511) a size of a gap betweenframes on a second media web (150-2).

The present examples may be implemented, at least partially, as acomputer program product stored on the precision repeat devices (203)and the difference engine (100), and may include a computer readablestorage medium comprising computer usable program code embodiedtherewith. The computer usable program code, when executed by aprocessor, may execute the processes described herein including themethods of FIGS. 4 and 5.

The specification and figures describe a difference unit forsynchronizing between two web presses. The difference unit may include afirst sensor to identify a position of a first mark printed on a firstweb output from a first web press, a second sensor to identify aposition of the first mark of the first web, a third sensor to identifya position of a second mark printed on a second web, and asynchronization engine to determine a synchronization difference betweenthe first web and the second web based on the identification of thefirst mark and second mark as sensed by the second sensor and the thirdsensor.

This difference unit and associated systems and methods permit thesynchronization of two print streams. Because the correction is splitbetween two presses, distortion of each print stream is minimized. Forvery large print jobs, two print streams may be run in parallel,effectively doubling the print speed compared to a single press system.Further, the difference unit may be implemented in legacy printingpresses resulting, making sales of multiple-press systems with thedifference unit less difficult.

The preceding description has been presented to illustrate and describeexamples of the principles described. This description is not intendedto be exhaustive or to limit these principles to any precise formdisclosed. Many modifications and variations are possible in light ofthe above teaching.

What is claimed is:
 1. A difference unit for synchronizing between twoweb presses, comprising: a first sensor to identify a position of afirst mark printed on a first web output from a first web press; asecond sensor to identify a position of the first mark of the first web;a third sensor to identify a position of a second mark printed on asecond web; and a synchronization engine to determine a synchronizationdifference between the first web and the second web based on theidentification of the first mark and second mark as sensed by the secondsensor and the third sensor.
 2. The difference unit of claim 1,comprising a communication engine to send the synchronization differenceto the first web press and the second web press.
 3. The difference unitof claim 1, wherein the first and third sensors are located at the sameposition along a web movement direction.
 4. A system for synchronizingtwo web presses, comprising: a first web press to output a first web,the first web press comprising a first precision repeat device; a secondweb press to output a second web; the second web press comprising asecond precision repeat device; and a difference unit comprising: afirst sensor to identify a position of a first mark printed on the firstweb; a second sensor to identify position of the first mark of the firstweb; a third sensor to identify a position of a second mark printed onthe second web; and a synchronization engine to determine asynchronization difference between the first web and the second webbased on the identification of the first mark and second mark as sensedby the second sensor and the third sensor.
 5. The system of claim 4,wherein: the first precision repeat device comprises: a first timer tomeasure a time T1 from a fourth sensor sensing the first mark and afifth sensor sensing a third mark printed on the first web; a secondtimer to measure a time T2 from the fifth sensor sensing the third markand the fourth sensor sensing a next first mark; and a first controllerto control a gap between the frames by regulating when frames areprinted on the web based on T1 and T2; and the second precision repeatdevice comprises: a third timer to measure a time T3 from a sixth sensorsensing the second mark and a seventh sensor sensing a fourth markprinted on the second web; a fourth timer to measure a time T4 from theseventh sensor sensing the fourth mark and the sixth sensor sensing anext second mark; and a second controller to control a gap between theframes by regulating when the frames are printed on the web based on T3and T4.
 6. The system of claim 5, wherein: the synchronization enginecommunicates the synchronization difference to the first controller andthe second controller; and the first controller and the secondcontroller control their respective gaps based at least partially on thesynchronization difference.
 7. The system of claim 4, comprising afinishing device downstream from the difference unit to perform at leastone post-synchronization operation on the first and second webs.
 8. Amethod for synchronizing two web presses, comprising: with a firstprecision repeat device of a first web press, dynamically adjusting asize of a gap between frames on a first media web; with a secondprecision repeat device of a second web press, dynamically adjusting asize of a gap between frames on a second media web; with a differenceengine: identifying, with a first sensor, a position of a first markprinted on the first media web; identifying, with a second sensor, theposition of the first mark; identifying, with a third sensor, theposition of a second mark printed on the second media web; determining asynchronization difference between the first media web and the secondmedia web based on the identification of the first mark and the secondmark as sensed by the second sensor and the third sensor; and sending asynchronization difference to the first web press and the second webpress.
 9. The method of claim 8, comprising adjusting a gap between thefirst web and the second web based on the synchronization difference.10. The method of claim 9, comprising with a first controller of thefirst web press and a second controller of the second web press,controlling respective gaps between frames of the first web and thesecond web, respectively, based at least partially on thesynchronization difference.
 11. The method of claim 8, whereindetermining the synchronization difference between the first web and thesecond web based on the identification of the first mark and third markas sensed by the second sensor and the third sensor comprises: comparinga difference between: a time T1 between the first sensor identifying theposition of the first mark and the second sensor identifying theposition of the first mark; and a time T2 between the second sensoridentifying the position of the first mark and the third sensoridentifying the position of the third mark; and determining if T1 doesnot equal T2.
 12. The method of claim 11, wherein: when T1 is greaterthan T2, decreasing the gap between printed frames of the first web andprinted frames of the second web; and when T2 is greater than T1,increasing the gap between the printed frames of the first web and theprinted frames of the second web.
 13. The method of claim 8, wherein,with the first precision repeat device of the first web press,dynamically adjusting the size of the gap between frames on the firstmedia web comprises: measuring a third time T3 between a fourth sensorsensing the first mark printed on the first web and a fifth sensorsensing the second mark printed on the first web; measuring a fourthtime T4 between the fifth sensor sensing the second mark and the fourthsensor sensing a next first mark; and adjusting a gap between printedframes when T3 does not equal T4.
 14. The method of claim 8, wherein,with the second precision repeat device of the second web press,dynamically adjusting the size of the gap between frames on the secondmedia web comprises: measuring a fifth time T5 between a sixth sensorsensing the third mark printed on the second web and a seventh sensorsensing a fourth mark printed on the second web; measuring a sixth timeT6 between the sixth sensor sensing the third mark and the seventhsensor sensing a next third mark; and adjusting a gap between printedframes when T5 does not equal T6.
 15. The method of claim 12, whereindecreasing the gap between printed frames comprises reducing an amountof time between printing sequential frames on a media web; andincreasing the gap between printed frames comprises increasing theamount of time between printing sequential frames on a media web;wherein adjusting a gap between printed frames comprises determining anerror in timing between sensing the first and second marks.